This application relates to in situ recovery of shale oil, and more particularly, to techniques for minimizing any effect on gas flow resistance in an in situ retort caused by a tendency of higher grade oil shale to expand upon heating more than does lower grade shale.
The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the hydrocarbon liquid product is called "shale oil".
One method for recovering shale oil is to form an in situ retort in a subterranean formation containing oil shale. Oil shale formation within an in situ retort site is fragmented to form a retort containing a fragmented permeable mass of formation particles containing oil shale. Formation particles at the top of the fragmented mass are ignited to form a combustion zone, and an oxygen-supplying gas, such as air, is supplied to the top of the fragmented mass for sustaining the combustion zone and for advancing the combustion zone downwardly through the fragmented mass. As the combustion zone advances through the fragmented mass, hot processing gas forms a retorting zone on the advancing side of the combustion zone. In the retorting zone, kerogen in the formation particles is decomposed to produce shale oil and gaseous products. Thus, a retorting zone moves from top to bottom of the fragmented mass in advance of the combustion zone. The shale oil and gaseous products produced in the retorting zone pass to the bottom of the fragmented mass for collection.
U.S. Pat. No. 4,043,595, which is assigned to the same assignee as this application, discloses a method for explosively expanding formation containing oil shale to form an in situ oil shale retort. That patent is incorporated herein by this reference. According to a method disclosed in that patent, an in situ retort is formed by excavating formation to form a columnar void bounded by unfragmented formation having a vertically extending free face, drilling blasting holes adjacent the columnar void and parallel to the free face, loading the blasting holes with explosive, and detonating the explosive. This expands the formation adjacent the columnar void toward the free face in layers severed in a sequence progressing away from the free face so that fragmented formation particles occupy the columnar void and the space in the in situ retort site originally occupied by the expanded shale prior to such explosive expansion. The void fraction or void volume in the fragmented mass corresponds to the volume of the columnar void formed before explosive expansion. The void fraction in the resulting fragmented mass is determined by the volume of formation removed from the retort site to form a void space toward which unfragmented formation remaining in the retort site is explosively expanded, inasmuch as such unfragmented formation is fragmented and expanded to fill such a void space. The original void volume is essentially distributed between the fragmented formation particles in the retort being formed.
Oil shale deposits occur in generally horizontal beds, and within a given bed there are an extremely large number of generally horizontal deposition layers containing kerogen known as "varves". The kerogen content of the formation is typically non-uniformly dispersed throughout a given bed.
The average kerogen content of formation containing oil shale can be determined by a standard "Fischer assay" in which a core sample customarily weighing 100 grams and representing one foot of core is subjected to controlled laboratory analysis involving grinding the sample into small particles which are placed in a sealed vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton", referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as in the Fischer analysis.
The average kerogen content of formtion containing oil shale varies over a broad range from essentially barren shale having no kerogen content up to a kerogen content of about 70 gallons per ton. Localized regions can have even higher kerogen contents, but these are not common. It is often considered uneconomical to retort formation containing oil shale having an average kerogen content of less than about 8 to 10 gallons per ton.
Formation containing oil shale that is suitable for in situ retorting can be hundreds of feet thick. Often there are strata of substantial thickness within such formation having significantly different kerogen contents than other strata in the same formation. Thus, for example, in one formation containing oil shale in Colorado that is a few hundred feet thick, the average kerogen content is in the order of about 17 gallons per ton. Within this formation there are strata 10 feet or so thick in which the kerogen content is in excess of 30 gallons per ton. In another portion of the same formation there is a stratum almost 30 feet thick having nearly zero kerogen content. Similar stratification of kerogen content occurs in many formations containing oil shale.
As described above, during the course of retorting, hot retorting gas flows downwardly through the fragmented mass of formation particles in an in situ retort. The void fraction of the fragmented mass influences the resistance of the fragmented mass to such gas flow. A fragmented mass with a high void volume has low resistance to gas flow, while a fragmented mass with low void volume has a high resistance to gas flow. Flow resistance of the fragmented mass is important inasmuch as retorting may be continued for an extensive period of time. For example, one experimental in situ retort a little over 80 feet high was retorted over a period of 120 days. If there is a high resistance to gas flow, a relatively high pressure drop will occur along the length of the fragmented mass. As a result, the blowers or compressors used for inducing gas flow will operate at relatively high pressure (for example, 5 psig) which requires appreciably more energy for driving the compressor or blower than if the pressure drop is relatively low. The total energy requirements can be relatively high because of the long time required for retorting. Higher pressure operation also can take a greater capital expenditure for blowers or compressors, and some gas leakage from the retort can occur, further reducing efficiency.
The pressure differential or pressure drop from the top to bottom for vertical movement of gas down through the fragmented mass in an in situ oil shale retort depends upon various parameters of the retort and retorting process such as lithostatic pressure, void fraction of the fragmented mass, particle size in the fragmented mass, the temperature pattern of the retorting and combustion zones, gas volumetric flow rates, grade of oil shale being retorted, rate of heating of the fragmented mass, gas composition, gas generation from mineral decomposition and the like.
Papers relating permeability of a fragmented permeable mass of formation particles containing oil shale, and thus pressure drop across the fragmented mass, to various retort and retorting process parameters include "Prediction of the Permeability of a Fragmented Oil Shale Bed During In Situ Retorting With Hot Gas," by R. B. Needham, Paper No. SPE 6071, presented at 1976 Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME; "Some Effects of Overburden Pressure on Oil Shale During Underground Retorting," by G. W. Thomas, Paper presented at Society of Petroleum Engineers 1965 Annual Fall Meeting; "Structural Deformation of Green River Oil Shale as It Relates to In Situ Retorting," by P. R. Tisot and H. W. Sohns, (Washington) U.S. Department of Interior, Bureau of Mines (1971); and "Permeability Changes and Compaction of Broken Oil Shale During Retorting," by Edward L. Burwell, Samuel S. Tihen and Harold W. Sohns, (Washington) U.S. Bureau of Mines (1974). Each of these papers is incorporated herein by this reference and a copy of each of these papers accompanies this application. These papers indicate that the permeability of a fragmented permeable mass of oil shale particles tends to decrease and thus pressure drop across the fragmented mass tends to increase as overburden pressure increases, as grade of oil shale being retorted increases, as the temperature of the fragmented mass increases up to 800 degrees F., and as the average particle size of the fragmented mass decreases.
It is also desirable in forming an in situ retort to keep the total void volume as low as possible because of the cost of minimg to form a void into which formation containing oil shale is expanded. Further, when the void is formed in the retort site, removed formation either must be retorted by more cumbersome and polluting above-ground techniques, or the shale oil is lost when the mined out material is discarded.
Thus, the operator of an in situ oil shale retort is faced with opposing economic considerations that should be optimized. On one side is the cost and loss of total yield of the retort by mining out formation to create the void volume for the fragmented mass. On the other side is the cost of energy and equipment for forcing the retorting gas through the fragmented mass.